In media such as valuable papers including cash vouchers and prepaid cards, identity cards including licenses, etc. which should be anti-counterfeit, there have been recently used a micro character, a copy guard pattern, an infrared-light absorbing ink, a fluorescent ink and so on, in order to improve security. The fluorescent ink is an ink including a fluorescent material which cannot be almost viewed under visible light, and can be viewed when invisible light (ultraviolet light or infrared light) is irradiated. With the use of such a fluorescent ink, there can be formed, on a valuable paper or the like, a fluorescent image (light-emitting image) which appears only when invisible light within a specific wavelength range is irradiated. Thus, it is possible to prevent that the valuable paper is easily forged by a generally used color printer or the like.
In addition, in order to further improve the anti-counterfeit effect, there is proposed that a light-emitting image, which cannot be viewed by the naked eye, is formed on a valuable paper by means of a fluorescent ink. For example, Patent Document 1 discloses a medium including a light-emitting image formed by using a first fluorescent ink and a second fluorescent ink. In this case, when seen with the naked eye, the first fluorescent ink and the second fluorescent ink are viewed as inks of the same color with each other, under visible light and ultraviolet light. On the other hand, when seen through a judging tool, the first fluorescent ink and the second fluorescent ink are viewed as inks of different colors from each other. Thus, the light-emitting image formed on the valuable paper cannot be easily forged, whereby the anti-counterfeit effect through the fluorescent inks can be enhanced. However, if there is a slight color difference or a thickness difference between the first fluorescent ink and the second fluorescent ink, there is a possibility that, when seen through the naked eye, the first fluorescent ink and the second fluorescent ink are not viewed as inks of the same color, whereby the light-emitting image is viewed.
Patent Document 1: JP4418881B